Jonny Evans would not, in any way, shape or form, be a vast improvement on what we have now nor is he good enough to be at a club like Arsenal.
There is a reason why United shipped him off.
He's good with the ball at his feet and can play out from the back. That's literally all he would be good for in the Arsenal team. He isn't good in the air. He isn't particularly strong (Imagine him against Ibrahimovic, he would get absolutely obliterated). He can be suspect positionally as well.
The move would make no sense. And do we seriously want another United cast-off?
Wenger truly has lost his mind if he is even contemplating this.